1. Field of the Invention
This invention relates generally to a process for improving the properties of raw or "green" cokes obtained by known processes from materials of petroleum origin and particularly to a process for desulfurizing and calcining such cokes without substantially lowering their bulk densities. As used herein, the phrase "without substantially lowering the bulk density" refers to the value of the bulk density of the final product of the process of the invention (desulfurized calcined coke) in relation to the bulk density of the same feed material (raw petroleum coke) after conventional calcination.
The major source of industrial petroleum coke originates in the delayed coker, and is produced at temperatures of about 900.degree. F. (482.degree. C.) by methods well known in the art. Unfortunately, many petroleum cokes produced by this method and other known methods contain appreciable amounts of sulfur, and cannot be directly utilized in the fabrication of carbon products due to this impurity. Aluminum producers, for example, the largest consumer in total quantity of calcined petroleum coke, demand low sulfur coke to satisfy pollution control requirements. It is therefore imperative that an economical process be available to bring about a substantial reduction in the sulfur content of these cokes, desirably to a level below 2 wt.%, and preferably to a level below 1.5 wt.%.
Raw petroleum coke for industrial purposes is conventionally calcined at temperatures in the range of about 1150.degree.-1300.degree. C. by methods well known in the art to remove a major portion of the volatile matter content of the coke and to provide increased density and conductivity therefor. During substantially complete calcination, the volatile matter content of petroleum coke is commonly reduced to below 1 wt.% and preferably below 0.5 wt.%. It is known that the customary temperatures utilized for calcination are not sufficiently high to bring about desulfurization of the coke.
A physical property of calcined petroleum coke recognized in the art as useful for determining the quality of the coke is bulk density, which is the weight per unit volume of coke particles having a defined size range. This value is commonly expressed in pounds/cubic foot or grams/100 cc. It is known that the bulk density of calcined coke must be maintained as high as possible to provide desirable properties, such as high strength, for products made from the coke. For example, the strength, reactivity and consumption rate of carbon anodes used in the electrolytic production of aluminum are directly related to the bulk density of the calcined petroleum coke used in the fabrication of such anodes. A reduction of more than about 10 percent of the bulk density of conventionally calcined coke will substantially affect the properties of a carbon product containing such coke.
2. Description of the Prior Art
It is known to desulfurize raw petroleum coke by directly heating the coke in a single stage to a temperature above about 1500.degree. C. in a rotary kiln or the like. Experience has taught that while this procedure effectively reduces the sulfur content of the coke, the bulk density and other physical properties are substantially deteriorated during the desulfurization process, as compared to the coke properties after calcination at conventional temperatures.
In the past, many staged processes have been developed for desulfurizing petroleum coke, particularly high sulfur fluid coke.
One method known in the art for the step-wise desulfurization of petroleum coke includes at least one stage wherein the coke is treated with hydrogen gas at elevated temperature. U.S. Pat. Nos. 2,721,169; 2,812,289 and 3,007,849 generally disclose such method for desulfurizing fluid coke. This method of treatment is expensive due to the relatively long treating time required and the cost of the hydrogen. Additionally, it is known that a hydrogen treatment of petroleum coke at elevated temperatures has a detrimental effect on various physical properties of coke, particularly the bulk density, compared to coke properties after conventional calcination.
Another step-wise method for desulfurizing petroleum coke generally comprises heating the coke at temperatures such that essentially all the volatile matter is removed therefrom and then heating the fully devolatilized coke at temperatures such that essentially all of the sulfur is expelled therefrom separately from the volatile components. U.S. Pat. Nos. 2,743,218 and 2,819,204, and British Pat. No. 755,061 generally disclose such a sulfur removal process. However, it has been discovered that removal of all the volatile matter of petroleum coke at an elevated temperature before further heating the coke to a temperature sufficient to desulfurize the coke produces a substantial reduction in the bulk density of the coke, compared to the bulk density of the coke after conventional calcination. Additionally, an appreciably lower amount of external fuel is required to heat the volatile-containing coke from the first stage of the process of the instant invention to desulfurizing temperatures as compared to fully devolatilized petroleum coke.
U.S. Pat. No. 2,716,628 provides a process for desulfurizing petroleum coke wherein the coke is held in a heat-soaking zone for a period of about six to twenty hours at a temperature of 2500.degree. to 3000.degree. F. (1371.degree. to 1649.degree. C.), and then contacted with a fuel gas, preferably methane, in a cooling zone, the coke in the heat-soaking zone being contacted with gas from the cooling zone in the presence of oxygen, the oxidation of the coke being minimized by the preferential combustion of the fuel gas from the cooling zone. An excess of fuel gas beyond that required for combustion is utilized to provide a heat transfer medium between the zones. This process is prohibitively expensive due to the requirement of excess fuel gas, the necessity of a source for the oxygen-containing gas and the time required for the heat-soaking treatment. It is known that treating petroleum coke with an oxygen-containing gas at elevated temperature has a detrimental effect on its bulk density due to excessive burning of the carbon material, which burning causes a comparatively rapid devolatilization of the coke, thus increasing the porosity.
U.S. Pat. No. 3,369,871 provides a multi-stage process for fabricating a low sulfur metallurgical carbon product from green petroleum coke comprising heating the coke at a temperature of at least 300.degree. F. (149.degree. C.) while flowing an oxygen-containing gas thereover to reduce the sulfur content of the coke, heating the desulfurized coke at a temperature of at least 1600.degree. C. and preferably about 1800.degree.-3500.degree. C. or more, to partially graphitize the coke, cooling the partially graphitized coke to about 1000.degree. F. (538.degree. C.), and subjecting the cooled coke to oxidizing gases until its sulfur content is below 0.2%. The bulk density of the coke resulting from this process would be detrimentally affected by the oxygen-containing gas treatment for the reasons presented above. Additionally, the temperatures required for the partial graphitization stage usually entail the use of expensive specialized heating techniques (e.g., induction heating) and equipment.